A013726 a(n) = 21^(2*n + 1).

21, 9261, 4084101, 1801088541, 794280046581, 350277500542221, 154472377739119461, 68122318582951682301, 30041942495081691894741, 13248496640331026125580781, 5842587018385982521381124421, 2576580875108218291929075869661, 1136272165922724266740722458520501

Offset: 0

Author

N. J. A. Sloane

Keywords

• nonn
• easy

Links

• Vincenzo Librandi, Table of n, a(n) for n = 0..200
• Tanya Khovanova, Recursive Sequences.
• Index entries for linear recurrences with constant coefficients, signature (441).

Crossrefs

Bisection of A009965 (21^n).

Cf. A005408, A013708-A013725, A013727-A013729.

Programs

Magma

[21^(2*n+1): n in [0..15]]; // Vincenzo Librandi, Jun 26 2011

Maple

seq(21^(2*n+1),n=0..10); # Nathaniel Johnston, Jun 25 2011

Mathematica

Table[21^(2 n + 1), {n, 0, 15}] (* Wesley Ivan Hurt, Feb 10 2014 *)

PARI

a(n)=21^(2*n+1) \\ Charles R Greathouse IV, Jul 11 2016

Formula

From Philippe Deléham, Nov 28 2008: (Start)

a(n) = 441*a(n-1); a(0)=21.

G.f.: 21/(1-441*x). (End)

a(n) = A009965(A005408(n)). - Wesley Ivan Hurt, Feb 10 2014

From Elmo R. Oliveira, Jul 10 2025: (Start)

E.g.f.: 21*exp(441*x).

a(n) = A013708(n)*A013712(n). (End)